1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with producing tones with time variant harmonic strength by convoluting two signal spectra.
2. Description of the Prior Art
An elusive goal in the design of keyboard electronic musical instruments is to attain the ability to realistically imitate the easily recognizable sounds of conventional acoustic type musical instruments. It has long been recognized that, with the notable exception of conventional organ tones, almost all tones produced by acoustic musical instruments exhibit tone spectra which are time variant in composition. A simple tone having a waveshape that is repeated cyclically and endlessly quite rapidly fatigues a listener.
The most commonly used tone generation system employed to produce time variant harmonic tone structure is the generic system called a "synthesizer." A synthesizer system contains a sliding formant filter as its essential constituent. The sliding formant filter is generally implemented as a frequency domain filter of either the low pass or high pass type and is configured so that it has the capability of varying the filter cut-off frequency in response to an electrical control signal.
Analog tone synthesizers usually employ a voltage controlled frequency filter to provide a sliding formant frequency response. Such a filter can vary the analog musical waveshape spectral response under the action of a control signal. Digital tone generators can produce corresponding spectral variations, similiar to those obtained in analog systems, by employing digital filters to vary the spectral content of a digital sequence of waveshape points which are later converted into analog signals to furnish a musical waveshape. A digital filter implementation for use as a formant filter subsystem is described in U.S. Pat. No. 4,267,761 entitled "Musical Tone Generator Utilizing Digital Sliding Formant Filter."
Synthesizers employing sliding formant filters are given the generic designation of subtractive synthesis tone generators. This terminology is appropriate because a sliding formant filter acts only to reduce the strength of frequency components which are already present at the input terminals of the filter. No new harmonic components are produced if the formant filters are linear system elements.
An alternative to the use of sliding filter formants to produce time variant harmonics in a musical tone is to employ a time variant nonlinear transformation of a musical waveshape. An example of such a waveshape distortion system is disclosed in U.S. Pat. No. 4,300,432 entitled "Polyphonic Tone Synthesizer With Loudness Spectral Variation." A combination of waveshape distortion and a sliding formant filter is disclosed in U.S. Pat. No. 4,300,434 entitled "Apparatus For Tone Generation With Combined Loudness And Formant Spectral Variation."
A common technique for distorting a musical waveshape is to use some form of signal modulation. Musical instruments have been designed which employ frequency modulation subystems to produce time variant distortions of a simple sinusoid signal at the musical tone's fundamental frequency. Such a system is disclosed in U.S. Pat. No. 4,175,464 entitled "Musical Tone Generator With Time Variant Overtones." Experimentally it has been found thatthe most useful musical effects are obtained when the modulation frequency is close to or equal to the carrier frequency.